Electrodynamic recording head



July 1l, 1950 H .mil

Filed Aug- 25, 1947 C. R. DOUGLASS, JR

ELECTRODYNAMIC RECORDING HEAD Patented July 11, 1950 UNITED STATES PATENT oFFicE '4 Claims;

(Granted under the act oi March: 3, 1883,:-as

amended April-3,0, 1928;: 370 0. G. 757) Thisinventi'on relatesingeneral to transducers and: more particularly: to a recording headv for thealateral cut disci type of phonograph record.

The recording head? ofk this vinvention is of the magnetic type, but as distinguished:'fromy those ofthe prior art, ithas only aziraction of the mass of those in commonv use. The` discovery ofthe structure-of this inventionthatenables a material reduction in the massof the-.electromagnetic unit' giveszrise toa number of advantages of the'unit of thisinventionloverthoseoi the .prior art.

Accordingly, it isy an object. voi? this invention to' provide a. recording unit` of: the magnetic' type in which the moving mechanism has a fraction of the mass of the moving mechanism of vthe magnetic recorders-ofthe prior art.

Another object isv to provide a recorder with a reduction in the inertia ofthe moving mechanism which results in the use of less stii^-spring centering and returnL mechanism.

Another object :isato'y provideta-,recorder with a moving mechanism that requires less dampingv and/results in an overallincreasgein efiiciencyofl the moving system.

Another object is to provide a recorder with a.- moving mechanism that displaces Ythe mechanicallyA resonant peak above 'the useful frequency range of the recording head.

Another object is tofprovide a .recorder that has less transient distortion due to the reduction of inertia.

Another object is toprovide a recorder in which the harmonic distortion has beenureduced to Va greatv degree over that present in magnetic .ren corders of theprior art.

Another objectv is to `provide a hermetically sealedrecorder; inwhich fluid damping. is'utilized'.

Another object is the provision of a-recorder Which Willbe economicalt to manufacture, reliable in operation and which possesses all ofthequalities of ruggedness and dependability. in service.

Figure 1Y illustrates affront` elevation viewof'the recording unit.

vFigure ZiHuStrates a side elevation view of the.

recording unit.

Figure 3, illustrates .a plan. view of the `recording unit.

The recording head of this inventionl is of a different structure from that of'any other recording headt-hat has beenusedtor the purposesof recording sounde on disc. It embodies .many improvements that' have been made to overcome ob; jections to the magnetic` type of recording head.

The massk of the moving," mechanism is only a fraction. of the mass of; the magneticL type head that is in common use. This reduction. inmass, reduces, theA inertia of the; moving, system whichv results inthe use ofa much less stiff spring centering andl return system. This reductionv iny mass,l also reduces the amount of damping required which greatly increases the overall efficiency ofthe moving system. The use of this inghead, Where magnetic saturationA takesvv placeI Ili iS- in-smail-moving armatures and pole-pieces. here pointed out, and' will .be described indetail' later, that the type of moving coil used in the recording head of this invention is` different from the moving coil of` the prior art, andfthatit has several-l` advantages over the conventionalY moving coil system.

The recording head ofthi-s inventionl is hermetical'lly. sealed and instead of 'using' the conventional rubber or gelatin composition for a damping material; whose characteristics vary considerablydue-to temperature changes, a fluid' is used for damping. This fluid'is capable oi withstanding Wide temperature variations Withouty changing vitsV Visco-sity. The case is'iled to twoeth-irdsof its height Withv fluid" of Vproper visccsity-to offer the required damping for proper recording response. The main cutting needle actuating shaft 'l in Figure2, -projectscut in front ofthe case,'thereby allowingythe cutting `needle to operate in a position AWithout obstructions on either side. This is an importantv advantage, for the vacuum shaving collection tube 20, which is'used toy pick up shaving I8 cu-t'by needle IT, is mounted adjacent to the needle. This is an improvement that allows greater dependability in collecting this shaving, which has proven-to be a hazard With recording heads of conventional construction, Where the cutting needle projects from the bottom of'the cutting head` In many currentlyl used recording heads the needle protrudes through the bottom of thev case.

The-needle is usually lsurrounded by a rubber these small pieces of foreign material1 rWork upl into the opening during needle replacement. This difficulty is overcome in the recording head of this invention due to the needle chucking device being completely on the outside of the case.

Referring to Figures 1 to 3, the recording head receives its audio excitation from the proper source impedance or terminals,` 2l. lThese exciting currents flow through coil I which is an extremely light insulating composition bobbin` wound with the required number of turns of wire. This coil is oblong in shape as shown at 22, so that the greatest percentage of turns shall be in the magnetic` field. There are two complete magnetic circuits, one generated by magnet I3 and one by magnet lll. The magnets are poled so that the flux paths I5 and I6 will be as shown, which Will result in additive lateral forces being exerted by eachiside of coil I. Coil I is mounted on aluminum alloy pins E which are supported by main drive shaft 1, Main drive shaft 'I is pivoted in two places by hardened steel wire pivot springs 8. Steel pivot springs 8 are vimbedded rmly in cleats 9. Thismethod of suspension in itself is unique, for the centering and stiifening springs il are used as the pivoting medium also. The length of springs 8 isadjusted duringmanufacture to oifer the required mechanical stiffness to the moving system. The cutting needle I'I is chucked in the main shaft 'I by means of screw 26. The turning motions of the main shaft are hereby translated to lateral motions at the tip of the cutting needle II, which further reduces the mass of the moving system by eliminating the usual heavy lower needle -chucking device. The helmetc seal between the main shaft 'l and front plate I2 of the case is effected by a conical rubber gasket IU which is at all times pressed in by spacing washer 2l'. All mechanism in the record.- ing head is bolted to the front plate l2 which in turn is bolted to the cast aluminum case II, mounted on the tone or carrying arm by the tapped holes 28.

A' number of advantages have resulted in using the recording head of this invention. In the past several years, especially during war time, many thousands of hours of disk recording have been made both in the studio and in the field. Much difficulty has been encountered in recording high fidelity records, especially in the field. These difficulties can be traced primarily to the recording heads used` The heads used in the field in the past have suffered variations in frequency response and efliciency due to the wide range of temperature and humidity changes encountered. Furthermore, in order to record high fidelity with currently used recording heads, an audio emplifier power output of 50 watts is required. By using the recording head of this invention, high ndelity response can be had with a power output of 12 watts, which effects a desirable saving of weight in the portable recording units.

Through the use of hermetic sealing in the recording head of this invention, consistent high fidelity recording can be accomplished under conditions of varying temperature and humidity without danger of failure due to corrosion and fungus growths.

In operation, the currents varying in strength in accordance with the intelligence to be recorded are supplied through lead-in terminals 2l, which conduct the currents to coil I where they set up a magnetic iield whose strength varies proportionally to the strength of the currents. As pre- 4 viously pointed out, the permanent magnets are so poled as to produce the maximum amount of torque in the resulting reaction between the magnetic field of the permanent magnets and the magnetic eld set up by the movable coil in response to the varying currents received. The reactions of the magnetic fields tend to displace the coil from the air gap of the magnets, first on one side and then on the other, depending upon whether the currents and the corresponding magnetic field produced thereby are increasing or decreasing. This developed torque tends to rotate the coil about a pivot formed by the springs 8, and it also rotates the cutting needle I'I about said pivot. It is clear that the tip of the needle has a lateral motion, the amplitude of which is proportional to the strength of the currents supplied. When the moving needle is imbedded in a moving record, it cuts a groove whose lateral variations are proportional to the variations of the currents supplied. If the supplied currents carry intelligence such as a speaking voice, the record so made by said currents will reproduce the voice upon reproduction as a phonograph record.

The novel structure set forth in this inventio enables high fidelity recordings to be made under conditions where said recordings were runable to be made prior to this invention.

The invention described herein may be manufactured and used .by or for the Government of the United States for governmental purposes without the payment to me of any royalties thereon or therefor.

What is claimed is:

l. In a transducer of the electromagnetic type having a moving mechanism of low mass for use in high fidelity recording comprising, a pair of magnets each having pole pieces positioned to form a centrally located airgap, said magnets having opposite poles adjacent each other, bridge members on both sides of one pair of opposite poles for spacing said magnets, a coil adapted to be energized by the signal to be recorded positioned in said airgap, a horizontally positioned shaft, support pins depending vertically from said coil, said support pins being rigidly attached at their other ends to the aforementioned shaft, vertically positioned spring pins mounted between said shaft and said bridge members, said spring pins acting as pivot mountings for said shaft, and means for mounting a stylus at one end of said shaft.

2. A transducer similar to that of claim l, and a housing means enclosing said magnets, coil and shaft, said housing means having an opening therein through which the shaft supporting the stylus enters.

3. A transducer similar to that of claim 1, and a fluid-tight housing means completely enclosing said magnets, coil and shaft, said housing means having an opening therein through which the shaft supporting the stylus enters, said opening having a conical rubber gasket associated therewith to eifect a hermetic seal between the shaft and the housing means, the housing means being filled with liquid to aid the damping of the said transducer.

4. In a transducer of the electromagnetictype having a moving mechanism of low mass for 'use in high fidelity recording comprising a pair of magnets each having pole pieces positioned to form a centrally located airgap, said magnets having opposite poles adjacent each other, bridge members on both sides of one pair of opposite poles for spacing said magnets, an electrodynamic Winding positioned in said airgap, a horizontally positioned shaft, supporting means for said winding located normal to said shaft and integral therewith, spring pins positioned parallel to said supporting means and integral with said shaft, said pins being rigidly attached to said bridge members for free movement of said shaft and Winding, means at one end of said shaft for mounting a stylus, and a uid tight housing means completely enclosing said magnets, Winding and shaft, said housing means having an opening therein through which protrudes the end of said shaft to which the stylus is mounted, the

housing means being fined with uquid to aid the 15 335,290

damping of said transducer.

CHARLES R. DOUGLASS, JR,

REFERENCES CITED The following references are of record in the le of this patent: v

UNTIED STATES PATENTS Number Name Date 1,689,339 Harrison Oct. 30, 1928 1,760,527 Robinson May 27, 1930 1,831,787 Goudy Nov. 10, 1931 1,870,446 FoleyV Aug. 9, 1932 2,213,936 Thompson Sept. 3, 1940 FOREIGN PATENTS Number Country Date Great Britain Sept. 25, 1930 641,339 Germany Jan. 28, 1937 

